1. Field of the Invention
The present invention relates to an electronic ignition timing control system for internal combustion engines (hereinafter referred to as engines) for electronically determining the ignition timing of the engine.
2. Description of the Prior Art
In the past, distributors conventionally used for determining the ignition timing of engines have been so designed that in the electric contact unit employing a cam and points, the relative positions of the various elements are varied in accordance with the engine rotational speed and the magnitude of the intake manifold vacuum respectively detected by the centrifugal governor and the vacuum advancer, thus generating an ignition signal at the proper ignition point corresponding to the operating conditions of the engine.
A disadvantage of this method is that all the necessary controls are effected mechanically, thus deteriorating the reliability of the ignition timing, making it difficult to provide an ideal ignition timing characteristic and giving rise to problems from the standpoint of exhaust emission controls which are now receiving attention.
Also an electronically controlled ignition system has been proposed in which the ignition timing is determined by dividing a predetermined ignition advance angle by the engine rotationl speed and converting into a time. A disadvantage of this prior art system is that since it is necessary to measure the engine rotational speed thus requiring a time for measuring the speed, the value of the engine rotational speed represents the average engine rotational speed within such measuring time, and consequently any change in the speed during the measuring time tends to cause an error. To overcome this difficulty, another system has been proposed in which a plurality of slits each corresponding to a predetermined crank angle are formed in the periphery of a disk, and the ignition timing is determined by detecting the proper slit. A disadvantage of this system is that in actual practice, if the interval between the slits corresponds to 2.degree. of crank travel and the disk is mounted on the crankshaft, it is necessary to form 180 slits in the disk, and consequently this system is not fit for use in practical application in consideration of the required processing capacity and the capacity and durability of a sensor for detecting the slits.